THE Andrew Mitchell "Plebgate" affair has deepened after it emerged police have arrested and questioned a second man.

The 23-year-old, who is not a police officer or member of police staff, was arrested on Wednesday night on suspicion of intentionally encouraging or assisting the commission of an indictable offence on or around December 14, said Scotland Yard.

He was questioned at a London police station and released on bail yesterday.

It remains unclear exactly what the arrest relates to, but the date of the alleged offence – December 14 – was one day after police received fresh information on "Plebgate" and a day before a police officer was arrested on suspicion of misconduct in public office.

Yesterday, police widened the investigation amid signs of strain between senior Conservatives and the police over the treatment of Mr Mitchell, as claims were made that officers conspired to fabricate evidence against the former Chief Whip, which ultimately led to his resignation in late October.

Prime Minister David Cameron expressed concern that an officer tried to "blacken the name" of Mr Mitchell, amid mounting questions over the initial account of his row with Downing Street police who refused to let him ride his bicycle through the main gates.

The officer is said to have e-mailed his local MP, Conservative John Randall, posing as a member of the public and accusing Mr Mitchell of calling police "plebs".

The account in the e-mail, written the day before The Sun first broke news of the row on September 21, was very similar to that in the police log, which was later leaked to the Daily Telegraph.

Scotland Yard said its investigation could look at the possibility of a conspiracy.